Multiple proteolytic events in caspase-6 self-activation impact conformations of discrete structural regions

Abstract

Caspase-6 is critical to the neurodegenerative pathways of Alzheimer’s, Huntington’s, and Parkinson’s diseases and has been identified as a potential molecular target for treatment of neurodegeneration. Thus, understanding the global and regional changes in dynamics and conformation provides insights into the unique properties of caspase-6 that may contribute to achieving control of its function. In this work, hydrogen/deuterium exchange MS (H/DX–MS) was used to map the local changes in the conformational flexibility of procaspase-6 at the discrete states that reflect the series of cleavage events that ultimately lead to the fully active, substrate-bound state. Intramolecular self-cleavage at Asp-193 evoked higher solvent exposure in the regions of the substrate-binding loops L1, L3, and L4 and in the 130s region, the intersubunit linker region, the 26–32 region as well as in the stabilized loop 2. Additional removal of the linker allowed caspase-6 to gain more flexibility in the 130s region and in the L2 region converting caspase-6 to a competent substrate-binding state. The prodomain region was found to be intrinsically disordered independent of the activation state of caspase-6; however, its complete removal resulted in the protection of the adjacent 26–32 region, suggesting that this region may play a regulatory role. The molecular details of caspase-6 dynamics in solution provide a comprehensive scaffold for strategic design of therapeutic approaches for neurodegenerative disorders.